Compact zoom lenses having a small number of lens elements and providing large zoom ratios are highly desirable.
Zoom lenses with a negative power front lens unit, a positive power middle lens unit and a negative power rear lens unit (i.e., "-+-" configuration) that provide 2.times. to 3.times. zoom ratios are known. Some of the zoom lenses of the "-+-" configuration are relatively compact and have relatively few lens elements (seven). U.S. Pat. No. 5,539,582 discloses such zoom lenses. However, these zoom lenses do not provide large enough zoom ratios and their structures can not be easily modified to provide larger zoom ratios.
Zoom lenses of the "-+-" configuration providing larger zoom ratios are also known. For example, U.S. Pat. No. 5,153,776 discloses zoom lenses with eleven lens elements providing zoom ratios of 3.6.times.. U.S. Pat. No. 5,289,317 discloses zoom lenses with ten and eleven lens elements. Some of these zoom lenses provide zoom ratios of 3.6.times. and 4.6.times.. All of these zoom lenses are large, complex, have many lens elements and, therefore, are expensive to produce.